1. Field of the Invention
The present invention relates to a method for the preparation of a lithium phosphate compound with an olivine crystal structure, which has a chemical formula of LixMyM′1-yPO4 wherein 0.1≦x≦1, 0≦y≦1, in particular to a method for the preparation of a nano-scale lithium phosphate ceramic powder by a self-propagating solution combustion.
2. The Prior Arts
The secondary lithium-ion battery has several advantages such as high energy density and superior life cycle, which has rapidly substituted the nickel-cadmium battery and nickel metal hydride battery after developed. The market of lithium-ion battery has increased consistently after the commercialized product was launched by Sony in 1991. The total production volume in more than 10 years was more than the summary of the nickel-cadmium battery and nickel metal hydride battery. The application area of lithium-ion battery has been expanded with the improvement or presence of new materials and battery technology. The 3C (Computer, Communication and Consumer) product has the properties of being light, thin, short and small, which makes the lithium secondary battery the best choice.
Lithium iron phosphate (LiFePO4) battery (LFP) is the new generation of lithium secondary battery, which has been attracting enormous research interest in vehicle, electric tools, and aviation industry. The commercialized product was launched in 2004, way behind the launches of nickel metal hydride battery in 1990 and lithium cobalt (the battery used in 3C products at present) in 1992. Lithium iron phosphate offers no safety problems of overheating or explosion, 4 to 5 times of cycle life and 8 to 10 fold of high power discharge (high power density, which can generate larger current suddenly) in comparison to general lithium-ion battery. In addition, the total weight at the same energy density of a lithium iron phosphate battery is 30-50% lower than that of other lithium-ion battery. Major corporations including Boeing, General Motors, Ford, Segway, and Black & Decker all are highly interested in development of Lithium iron phosphate battery.
Lithium iron phosphate batteries also have their drawbacks. The energy density of lithium iron phosphate batteries is 25-40% lower than that of LiCoO2, which is not applicable in portable 3C products with high energy density. In addition, the high threshold in powder sintering technology and difficulties in mass production of lithium iron phosphate battery have made it expensive to be used broadly in related industry.
The ceramic crystal of LFP has an olivine structure, a slightly twisted hexagonal close-packed structure which commonly exists among natural minerals. The artificial synthesized powder is used widely since LFP has very low purity in natural mineral olivines. The crystal structures of MO6 octahedra and PO4 tetrahedra limit the change in crystal lattice volume, which affects the insertion and extraction of lithium ions, further lowers the diffusion rate of lithium ions and causes the decrease of lithium ion electronic conductivity and diffusion coefficient. Therefore, artificial synthesis through decreasing the particle size or doping has become a key point for the recent research and development as well as the objective for the present invention.
The main synthesis methods for LFP include solid-state reaction, carbonthermal reduction method, hydrothermal synthesis and so on. These methods are briefly described below.
1. Solid State Reaction
In general, lithium salts, ferrous compounds and phosphate compounds are mixed and heated to react and yield lithium iron phosphate after diffusion. As mentioned in U.S. Pat. No. 5,910,382, Li2(CO3), Fe(CH3COO)2, and NH4H2PO4 were mixed according to the stoichiometric ratio and put into a high temperature oven, heated at 650-800° C. for 24 hours at the presence of inert gas. The LFP products were grinded into proper particle sizes. However, the method needs an excessively high temperature in long time, which is energy consuming and causes grouping of products. The distribution of product sizes is uneven after grinding, and the instrument can be contaminated easily. Therefore the method is of less economical value, and the poor quality of product is not suitable for mass industrial application.
The Taiwan Patent No. I292635 discloses an alternative method using a metallic crucible as the container of powder and carbonate salt as a reactant to generate a protective atmosphere in order to save the cost of inert gas. There are still drawbacks of energy consuming, uneven particles and contamination due to high temperature in long time.
2. Carbonthermal Reduction
The abovementioned solid state reaction using compounds with Fe2+ as reactant, which is more expensive than compounds with Fe3+. In order to solve the above problems, the precursors of carbon are generally added to reactants of lithium compound, Fe3+ compound and phosphate during the preparation to reduce Fe3+ to Fe2+ as mentioned in U.S. Pat. Nos. 6,528,033, 6,716,372, and 6,730,281. The amount of carbon is difficult to control in these methods though the cost of reactants could be decreased. Too little carbon will affect the characteristics in materials since Fe3+ could not be reduced, while too much carbon could result in reducing the iron compound to iron metal, followed by lowering the electronic capacity.
Another method in Taiwan Patent No. I254031 discloses heating a carbon source to generate fine carbon particles, then carrying these particles to reacting area by inert gas to reduce Fe3+ to produce Fe2+ to overcome the drawback mentioned above. However, the processes are more complicated, and are still time- and energy-consuming since the carbon source needs to be heated at 300° C. to decompose first, then reacted at 700° C.
3. Hydrothermal Synthesis
Hydrothermal methods have been applied to the synthesis of lithium iron phosphates by reacting soluble lithium compound, ferrous compound and phosphate salt under high temperature and high pressure in aqueous solution. Nano-scale lithium iron phosphate particles at even size of 0.5 μm were synthesized by reacting lithium hydroxide (LiOH), ferrous sulfate (FeSO4) and phosphate at 150-200° C. in hydrothermal condition, followed by treatment at 400° C. with nitrogen gas for several hours (Keisuke Shiraishi et al., Journal of Power Sources 146 (2005) 555-558). However, this study was limited to the academic field because of harsh synthetic condition, expensive equipments, and drawbacks of high cost as well as difficulties with mass production.